The illnesses were reported Friday at the University Drive campus in the city's Oakland section, one of three facilities in the VA Pittsburgh Health System.
The bacteria that cause the particular type of pneumonia have been found in the water supply. The hospital has a copper ionization system that's supposed to eliminate the bug, but it may not be working properly.
Legionnaire's is not contagious from person to person so clinical care hasn't been interrupted. People catch it by inhaling water tainted with the bacteria.
VA officials have reviewed the situation with the Centers for Disease Control and Prevention.